RTV is a good choice if your surface is well-prepared. We've used it.
Don't do a bead (warping because of different thermal expansion rates) - do point contacts. For theory, google "mirror cell plop" to calculate the mechanical strain of a mirror against a plate. Looks like for a .875in thickness blank the max diameter is 8.7in. So a 4.5 is about .5in thick. Try for 3 equidistant points - it would help to have one in the center, but when you try to remove it you're going to have a difficult time, so it's your call.
Clips are a bad idea anyway - use the cork you have if you can. But get off that old cement completely - they're incompatible glues. The cork probably rotted because it was layered between two nonporous surfaces, and cork is very hydrophiliac. My father-in-law would probably have put it in a low vacuum chamber for a few weeks, and done different pull tests to make sure that any one of the pad areas was big enough and strong enough to hold the mirror, but he worked on satellites at Ford Aerospace / Loral (see http://jolitz.telemuse.net/wljolitz.) :-)
An excellent solvent for RTV is eucalyptus oil, but don't get too much of it on your hands (poisonous in quantity) nor let it near children / pets (keep it locked up). You can get some cheaply at the "Herb Room" in Santa Cruz.
If you don't clean well enough, RTV doesn't adhere well and the weight of the mirror will cause sagging or (worse yet) fall off. RTV doesn't adhere well to enamels, BTW, so go to the metal surface. -l
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> -----Original Message-----
> From: sf-bay-tac-bounces@seds.org [mailto:sf-bay-tac-bounces@seds.org]On
> Behalf Of Mike J. Shade
> Sent: Wednesday, March 02, 2005 8:41 AM
> To: sf-bay-tac@seds.org
> Subject: [TAC] (OT) RTV and secondary mirror
>
>
> I'm going to reattach the 4.5" secondary mirror for the 12.5"
> Ritchey-Chretian. It looks like there were three cork pads,
> about 1.5" and
> these were glued to the back plate and the mirror was glued to
> these...I'm
> thinking that three buttons or RTV might be a better choice,
> about the same
> spacing but perhaps not quite as large...anyone have any thoughts on
> this? Might it be better to put a thin bead at the edge? More
> cork (which
> I suspect might have been stressing the mirror a bit)?
>
> Clips are out of the question...
>
> Thanks...BTW. the original cork pads are destroyed and it looked like the
> previous owner used windshield repair cement (that yellow looking stuff
> that dries hard)...it doesn't look like RTV...
>
>
> Mike J. Shade: mshade@theriver.com
> Director, Sonoita Hills Observatory, Sonoita Arizona
> www.sonoitaobservatories.org
>
> See work done at the observatory: http://c3po.cochise.edu/astro
> under the "images & photos" panel on the left of the page
>
> Fight wasteful and obtrusive outdoor nighttime lighting:
> International Dark Sky Association: www.darksky.org
>
> "I like the dark, it's cheap." Ebinezer Scrooge
>
>
>
Received on Wed Mar 2 10:26:01 2005